With respect to a few key story elements:
The reason no ship has returned from the Omega 4 relay is simple. First, the relay dumps you right on the edge of the accretion disc of a black hole - literally on the edge of the event horizon. Even with the advanced tech available in the 23rd century, apparently the event horizon still presents an insoluable problem - cross it and you can experience being pulled apart into your constituient sub-atomic particles.
The cause is simple - the Mass Relays have some amount of error involved when they drop a ship into place. According to the codex entries, this is proportional to the total mass in transit. A relatively low mass ship like an alliance frigate therefore has a lower amount of drift than a dreadnought or a fleet warping simultaneously. Bringing in a larger fleet could certainly resolve the issue of the collector ship presuming you can survive the entry.
Given that the collector ship is cruiser class and therefore containing presumably substantially more mass, the explantion that the Reaper IFF is necessary for the jump seems to hold up. In actual, modern military history, GPS was once rigged such that any non-military reciever was given an arbitrary error of 100m in order to deny its full utility to enemy forces. Once the technology was no longer soley in the hands of the US, the locks were lifted as one of the last acts of president clinton allowing for any properly equipped reciever to tringulate your position to an accuracy of 1m. This not only further ensures reapers maintain a critical edge in any future battles, it also allows for the approach to a base that is being used to apparently create additional reapers to be essentially impossible to navigate.
Still, it is called a suicide mission for a number of reasons. It is entirely possible to get your entire team killed (including shepard). You are sending ships through a relay that has never been explored - already a bad sign for survival rates. You are sending them against an unknown number of ships when one had demonstrated it was fully capable of having it's way with an Alliance Frigate. It is only through the use of adapted reaper, geth and advanced technology that the ship had enough of an edge to directly confront a single collector cruiser and even then the Normandy was disabled in the process. You are also sending your small team in against a dramatically superior force in terms of numbers. This is evidenced by the fact that you will lose people holding the line if you take too long killing the Reaper. As far as why you need ten people, in spite of the fact that one can only controll 3 at a time, I would simply point out that the others are presumably helping in ways you cannot imagine. One member must open doors, while the secondary fire team draws attention. In the second segment, your team progresses through a seeker swarm thick enough that it was likely presumed that route was secure leaving the majority of your force under fire from the collectors. What's more, on a ship that large, a truly organized response from defenders would take time to mount.
In short, you are sending people through a relay that had never been navigated and expecting it to face down at least one ship that had demonstrated it's power once already. Your team of 10 or 10 specialists is expected to fight their way through a space station potentially containing millions of Collectors in an effort to destroy the station. There is no hope of support of any kind - the only advantage you possess is surprise. Were I asked to undertake a similar mission, I'd certanily consider it suicide.
.